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NET_11.TXT
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1992-06-28
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NET_11.TXT
RADIO PATHS
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Before moving on to discussing backbone link arrangements, we need to touch
on the important, but oft-times neglected subject of RF links. Quite often
packet radio paths are just thrown together without performing anything other
than a "go-no-go" analysis. In squeezing the greatest amount of link coverage,
paths may end up with marginal performance. As the move to higher data speeds
is made, one needs to keep in mind the overall goal of reliability. The
network is no better than the proverbial chain with its weak link. One weak
radio link will create serious throughput problems and can render ineffective
an otherwise well engineered system.
At higher data speeds, cases will be found where marginal 1200 baud links are
unuseable due to the effects of multi-path (signal reflections). Or, by faster
data rates requiring higher received signal levels than does 1200 baud. As
amateurs, site selection will be heavily influenced by the kindness of existing
commercial or municipal site operators. Sometimes this will result in less
than optimum radio paths. When this occurs, link developers should keep in
mind possible alternative routes. Perhaps a friendly (non-packet) amateur near
the planned route would be willing to house a backbone relay node.
Where doubt is raised about the suitability of a radio path, it is possible
(and desirable) to perform radio path calculations to get an idea of future
link performance. This process is straightforward. One can calculate the
line-of-sight radio path margin by adding power and antenna gains. Then the
path loss is subtracted to find receive signal levels in dB. It's good
engineering practice to design paths to include a fade margin of 40 dB or
better. In other words, the received signal level should be at least 40 dB
greater than the receiver FM threshold.
Once calculations show promise, it's time to make a site survey and to measure
the link fade margin. This is easily done by receiving a signal from the
distant site with a calibrated step attenuator between the antenna lead and the
receiver. Attenuation is increased until the received signal begins to get
noisy. The amount of attenuation is then read and is the fade margin in dB.
This procedure should be repeated at the other end. If the fade margin is too
low, then options include: raising transmitter powers, increasing antenna
gains/heights, or looking for a better site.
In the process of going through the above steps, detailed topographic maps
should be consulted to get an indication of path terrain and obstructions.
With the knowledge gained, NodeOps will have a better insight into the
reliability of their radio links.
IBM compatible software programs exist for radio site evaluation. The more
comprehensive ones feature terrain elevation data on floppy or CD-ROM for path
profiles, intermodulation product analysis, plotter support for 3-D map
overlays, antenna coverage patterns, as well as point-to-point path loss and
fade margin calculations. The more sophisticated programs can be quite costly.
Some of the less complex programs are available as shareware. One such program
is free for radio amateur or non-commercial use. It is called PATHCALC and was
written by Jon Adams, NW6H. PATHCALC is a menu driven program and even those
inexperienced with computer operation or radio path calculations should have
little difficulty in using it. PATHCALC requires the user to know the latitude
and longitude of the site locations to the nearest minute. Knowledge of site
elevations in feet is also required. The program also allows the inputting of
equipment configuration losses and gains, if known.